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Multiplexed Quantum Light States from Nanowire Quantum Dots

Kaiser, Moritz

Published Jan 1, 2025• 0 citations
OpenAlex

Abstract

In this thesis, several optical excitation schemes for single-photon generation in nanowire semiconductor quantum dots were investigated. Fluctuations in the excitation pulse can degrade photon quality and reduce the reliability of the source. These limitations motivated the development of a new excitation scheme called chirped stimulated adiabatic rapid passage. It is designed to be robust against fluctuations in both the energy levels of the quantum system and the laser parameters, including pulse power, spectral detuning, and time delay. Building upon the principles of adiabatic rapid passage, the proposed approach employs an additional chirped stimulation pulse, yielding an enhancement in photon generation by a factor of 1.47 compared to adiabatic rapid passage. These features make the technique promising for the stable generation of single photons, with improved performance under realistic experimental conditions.

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In this thesis, several optical excitation schemes for single-photon generation in nanowire semiconductor quantum dots were investigated. Fluctuations in the excitation pulse can degrade photon quality and reduce the reliability of the source. These limitations motivated the development of a new excitation scheme called chirped stimulated adiabatic rapid passage. It is designed to be robust against fluctuations in both the energy levels of the quantum system and the laser parameters, including pulse power, spectral detuning, and time delay. Building upon the principles of adiabatic rapid passage, the proposed approach employs an additional chirped stimulation pulse, yielding an enhancement in photon generation by a factor of 1.47 compared to adiabatic rapid passage. These features make the technique promising for the stable generation of single photons, with improved performance under realistic experimental conditions.
To realize a scalable source of frequency-multiplexed single photons, it is important to have an ensemble of quantum emitters that can be collectively excited with high efficiency. Semiconductor quantum dots hold great potential here, due to their excellent photophysical properties. The most efficient scheme to excite a quantum dot ensemble employs chirped laser pulses, relying on the so-called adiabatic rapid passage, is due to its robustness against spectral and intensity fluctuations. Yet, the existing methods to generate chirped laser pulses coupled to a quantum emitter are bulky, lossy, less flexible, and mechanically unstable, which severely hampers the prospects of a practical quantum dot device. Here, we present a compact, robust, and high-efficiency alternative for chirped pulse excitation of solid-state quantum emitters. Our simple plug-and-play module consists of chirped fiber Bragg gratings (CFBGs) that provide high dispersion, tailored to ~ 800 nm with a ~ 5 nm bandwidth that accommodates the quantum dot spectral variability. To characterize and benchmark the performance of our method, we demonstrate the chirped excitation of a GaAs/AlGaAs quantum dot that shows excellent agreement with theoretical simulations and establish high-fidelity single-photon generation. Our method can be tailored for a wide spectral range and dispersion requirements and is a significant milestone toward realizing a direct fiber-coupled, plug-and-play quantum dot photon source.
Abstract Nanoscale bright sources that produce high-purity single photons and high-fidelity entangled photon pairs are the building blocks to realize high security quantum communication devices. To achieve high communication rates, it is desirable to have an ensemble of quantum emitters that can be collectively excited, despite their spectral variability. In case of semiconductor quantum dots, Rabi rotations are the most popular method for resonant excitation. However, these cannot assure a universal, highly efficient excited state preparation, due to the sensitivity to excitation parameters. In contrast, adiabatic rapid passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we show that the robustness of ARP holds true for the simultaneous excitation of the biexciton states in multiple, spatially separated and spectrally different quantum dots. For positive chirps, we also find a regime where the influence of phonons relax the sensitivity to spectral detunings and lower the needed excitation power. Being able to generate high-purity photons from spatially multiplexed quantum dot sources using the biexciton to ground state cascade is a big step towards the implementation of high photon rate, entanglement-based quantum key distribution protocols.
A scalable source of single photons is a key constituent of an efficient quantum photonic architecture. To realize this, it is beneficial to have an ensemble of quantum emitters that can be collectively excited with high efficiency. Semiconductor quantum dots hold great potential in this context due to their excellent photophysical properties. Spectral variability of quantum dots is commonly regarded as a drawback introduced by the fabrication method. However, this is beneficial to realize a frequency-multiplexed single-photon platform. Chirped pulse excitation, relying on the so-called adiabatic rapid passage, is the most efficient scheme to excite a quantum dot ensemble due to its immunity to individual quantum dot parameters. Yet, the existing methods of generating chirped laser pulses to excite a quantum emitter are bulky, lossy, and mechanically unstable, which severely hampers the prospects of a quantum dot photon source. Here, we present a compact, robust, and high-efficiency alternative for chirped pulse excitation of solid-state quantum emitters. Our simple plug-and-play module consists of chirped fiber Bragg gratings, fabricated via femtosecond inscription, to provide high values of dispersion in the near-infrared spectral range, where the quantum dots emit. We characterize and benchmark the performance of our method via chirped excitation of a GaAs quantum dot, establishing high-fidelity single-photon generation. Our highly versatile chirping module coupled to a photon source is a significant milestone toward realizing practical quantum photonic devices.
A quantum emitter (QE) is a physical system that can be used to encode a quantum state via some internal degree of freedom (e.g. exciton, electron spin, valley) and is coupled to light via a dipolar transition that enables the conversion of that quantum state into the state of a photon and vice versa. Such QEs can be applied to sources of single and entangled photons for applications in quantum cryptography or quantum imaging and a collection of coupled QEs can be used to realize a small quantum simulator or quantum memory node in a distributed quantum network. Among solid state QE systems, semiconductor quantum dots (QD) are particularly attractive due to their high radiative quantum efficiencies, their strong optical coupling, enabling fast and arbitrary qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. Adiabatic rapid passage (ARP) is a state preparation scheme that enables the excitation and de-excitation of a QE with a high fidelity even in the presence of fluctuations in experimental parameters such as intensity and frequency of the control laser pulse. This excitation scheme may be used for quantum state initialization in quantum information platforms and for triggering quantum light sources. This thesis work concerns an exploration of the efficacy of ARP for the quantum control of excitons in semiconductor QDs. The experiments carried out in this work show that the impact of phonons may be suppressed for ARP, pointing to the possibility of high-fidelity inversion at elevated temperatures. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the QD. The experiments performed in this thesis work also quantify the detuning tolerance of ARP. ARP was also used to demonstrate parallel quantum state initialization in two different QDs with different transition energies and dipole moments.
The generation of single photons using solid-state quantum emitters is pivotal for advancing photonic quantum technologies, particularly in quantum communication. As the field continuously advances toward practical use cases and beyond shielded laboratory environments, specific demands are placed on the robustness of quantum light sources during operation. In this context, the robustness of the quantum light generation process against intrinsic and extrinsic effects is a major challenge. Here, we present a robust scheme for the coherent generation of indistinguishable single-photon states with very low photon number coherence using a three-level system in a semiconductor quantum dot. Our approach combines the advantages of adiabatic rapid passage and stimulated two-photon excitation. We demonstrate robust quantum light generation while maintaining the prime quantum-optical quality of the emitted light state. Moreover, we highlight the immediate advantages of the implementation of various quantum cryptographic protocols.